The present invention pertains to processes for coating substrates with a metal compound. It is especially concerned with processes for depositing a thin metal compound coating containing a nuclear poison material on a fissile or fertile substrate.
Coatings of refractory metal compounds have been deposited on a variety of substrates by a variety of techniques. Some of the more commercially important coating technologies include Thermal Spray Coating, Chemical Vapor Deposition (CVD), Vacuum Coating, Sputtering, and Ion Plating. These technologies are summarized in the American Society for Metals, "Metals Handbook Ninth Edition: Volume 5 Surface Cleaning, Finishing and Coating," (published October, 1982) at pages 361-374 and 381-421. The aforementioned pages 361-374, 381 to 421 are hereby incorporated by reference.
Examples of CVD processes for coating a cemented carbide substrate with a refractory metal boride layer are described in U.S. Pat. No. 4,268,582. The specification of U.S. Pat. No. 4,268,582 is hereby incorporated by reference.
Thermal neutron fission reactors presently incorporate so-called "burnable poisons in the fuel or internal reactor structures. A "burnable poison" is an element which has a high thermal neutron absorption cross section, and which undergoes upon neutron absorption a nuclear reaction which transmutes the element to an isotopic species of much lower neutron absorption cross section. An example is the element Boron, which consists of two naturally occurring isotopes, .sub.5.sup.10 B and .sub.5.sup.11 B. .sub.5.sup.10 B has a very high (3,838 barns) cross section for the reaction ##STR1## whereas the product, .sub.3.sup.7 Li, has a very low neutron absorption cross section. During the operation of a thermal reactor, nuclei of the fissile fuel material (e.g. .sup.235 U) undergo fission under neutron bombardment. This process burns up fuel and also generates fission products which contribute residual neutron absorption cross sections which eventually act as a stable poison, reducing the reactor multiplication factor. Thus, in order to provide for long periods of reactor operation between fuel reloadings, it is desirable to incorporate at the start of each operational period, a fuel loading considerably in excess of that needed merely to bring the reactor to criticality and operate at the desired power levels. In order to help control the reactivity provided by the excess fuel and reduce the volume of control rods needed, a "burnable poison" such as the boron discussed above is incorporated. As the reactor operates, fuel is burned and reaction products build up. The resultant decrease in reactivity is offset by the disappearance of the burnable poison.
Some of the methods of incorporating burnable poisons, such as boron and gadolinium, into water reactor systems are described in J. T. A. Roberts, "Structural Materials in Nuclear Power Systems" (published 1981), at pages 53-60, and pages 119-124. Roberts, pages 53-60 and 119-124 are hereby incorporated by reference. Other methods of incorporating a burnable poison into the reactor system include coating the fuel pellets with the burnable poison in elemental or compound form.
Chemical vapor deposition techniques for coating fuel pellets with a burnable poison compound are disclosed in U.S. patent application Ser. Nos. 564,057 now U.S. Pat. No. 4,560,575 468,743 now U.S. Pat. No. 4,582,676; and 468,788 now U.S. Pat. No. 4,587,087, filed on Dec. 21, 1983, Feb. 22, 1983, and Feb. 22, 1983, respectively. Sputtering techniques for depositing burnable poison compounds coatings on fuel pellets are described in U.S. patent application Ser. No. 526,712, now U.S. Pat. No. 4,587,088, filed on Aug. 26, 1983. The foregoing patent applications are hereby incorporated by reference.
The present inventors have now surprisingly discovered a new process for applying a refractory metal compound coating to a substrate surface. The process according to the present invention requires that a metal compound be thermally decomposed in the vicinity of the substrate surface to be coated. Included in the products of thermal decomposition is the desired refractory metal compound which is deposited on the surface. Essentially, all other products of the thermal decomposition are volatilized.
The metal compound starting material contains a radical having one or more metals, and a second radical containing hydrogen and one or more of the elements oxygen, nitrogen, carbon and boron.
According to one embodiment of the present invention, the substrate surface to be coated has been preheated to a temperature above the thermal decomposition temperature of the metal compound. Preferably the temperature of the substrate is below about 400.degree. C.
According to another embodiment of the present a molecular beam containing the metal compound is impinged upon the surface to be coated. Upon impact the organometallic composition thermally decomposes. In this embodiment, while this substrate surface may be preheated, it is not required.